Resuspension and Diffusive Flux of Nitrogen and Phosphorus in a Hypereutrophic Lake

Abstract

AbstractBottom sediments in shallow lakes can play a major role in releasing nutrients to the overlying water column during wind induced sediment resuspension or by constant flux due to diffusion. Internal nutrient loads due to these processes may be equal to or higher than external loads. Laboratory and field experiments were conducted on Lake Apopka, a shallow, hypereutrophic subtropical lake located in central Florida. Ammonium (NH+4) and soluble reactive P (SRP) flux during sediment resuspension were measured under laboratory conditions using intact sediment cores. Ammonium N and SRP flux due soley to diffusion were assessed using in situ porewater concentrations. Average diffusive flux from sediment to the overlying water was estimated to be 25 mg NH4‐N m−2 d−1 and 1 mg P m−1 d−2. Resuspension fluxes of NH+4 and SRP were higher than diffusive flux. Soluble reactive P profiles of porewater showed distinct profile differentiation, with the surface 0 to 8 cm sediment depth acting as a P‐depletion zone, and the underlying sediment displaying steep gradients in porewater SRP. These results suggest that dissolved NH+4 and SRP transport from the surface 8 cm of sediment was due to sediment resuspension, while below this depth, upward mobility of NH+4 and SRP was regulated by diffusion. Although dissolved N and P flux is upwards (from sediment to water column), during extended periods (annual cycle) the lake is functioning as a net sink for N and P by transforming inorganic pools of nutrients into organic forms and depositing them on the sediment surface.